The present invention relates generally to filter apparatus for filtering electrical signals to permit only those signals in a selected band to pass. More particularly, the invention pertains to the field of broadband high pass microwave filters.
Microwave filter designs are based on prototype lumped element low frequency circuits. Filter elements at low frequencies behave as almost pure capacitances and inductances. At higher frequencies the inductances of connecting wires and the capacitance of metal conductors change the effective reactances and must be considered in the filter design. At microwave frequencies the wavelength becomes so short that the designer either must miniaturize the circuit elements or build the circuits using known transmission line concepts and wave properties. Filters with very wideband performance, however, cannot use transmission line filter designs with line lengths greater than a fraction of a wavelength because the effective line lengths (reactances) change radically from opens to shorts, i.e. from inductances to capacitances, and cause spurious pass or stop bands.
High pass prototype filter circuits have the form shown in FIG. 1. By way of example, a nine-element filter is illustrated. It is understood, however, that filters may have fewer or more elements depending on the performance requirements. The prototype filter illustrated in FIG. 1 is comprised of the string of capacitors C1, C2, C3, C4, and C5 and further includes the inductances L1, L2, L3 and L4 connected between the capacitors and ground as illustrated. Conventional semi-lumped microwave high pass filters implement the FIG. 1 prototype by using small capacitive discs separated by inductive shunt elements formed by high impedance lines. Often the assembly is made by using split-block coaxial circuits. The high impedance line length can be estimated by using the approximation EQU l = Lv/Z.sub.o ( 1)
where L is the inductance of the prototype filter element, v is the speed of light and Z.sub.o is the characteristic impedance of the wire, typically 150 to 200 ohms.
Another method often used to implement the prototype filter of FIG. 1 forms the capacitors by overlapping strip lines separated by dielectric blocks with the inductors formed by connecting strip lines between the overlapping capacitor sections. This technique, however, is useful only for a limited bandwidth because the total length of the capacitive assembly may be a significant fraction of a wavelength and because the capacitors and series connecting inductors also have parasitic capacitances to the ground plane.